This invention relates generally to measurement of root-mean-square (RMS) voltages, and in particular to extending the effective resolution and accuracy of a medium-resolution sampling RMS converter.
There are many types of converters for measuring the RMS value of an AC signal, and RMS measurement is now a common feature found on many digital multimeters. The RMS measurement accuracy provided by these conventional RMS converters is sufficiently adequate for general-purpose measurements. Some digital multimeters are optimized to measure the average value of sine waves, such as 60-Hertz power line signals, but are not accurate for non-sinusoidal waves. This has led to a distinction of xe2x80x9ctrue RMSxe2x80x9d as opposed to just xe2x80x9cRMSxe2x80x9d measurements.
High quality laboratory digital multimeters require highly-accurate precision measurement systems. For precision RMS measurements, thermal converters provide high accuracy, but are bulky and comparatively slow because they. rely on translation of RMS to heat. Sampling RMS converters and so-called delta-sigma that provide high accuracy are available, but are very expensive and again are comparatively slow because of the time required to resolve to 20 bits or more.
In accordance with the present invention, a precision RMS measurement system is provided by extending the effective resolution of a medium-resolution sampling RMS converter.
An AC or other time-varying cyclic signal is simultaneously applied to an average-responding AC-to-DC converter and to a high-speed medium resolution sampling analog-to-digital converter. The average voltage produced by the AC-to-DC converter is applied to a precision analog-to-digital converter, which in turn produces a highly accurate digital representation of the average value. The output of the sampling analog-to-digital converter is provided to a digital signal processor, which calculates the RMS and average values. A microprocessor then multiplies the ratio of the calculated RMS and average values by the highly-accurate DC average. The result is that the units for the average voltages cancel, leaving only the RMS units in a now highly-accurate RMS voltage. Thus, the resolution of the sampling analog-to-digital converter has been effectively extended. The precision RMS voltage is then displayed.
Since the ratio of the calculated RMS and average values is used, resolution error is resolved out in the division, and so the sampling analog-to-digital converter does not need to be calibrated to achieve full accuracy. Only the linearity is important.
One feature of the present invention is to provide the highest possible sampling rate without sampling the same points on the waveform over and over.